1. Field of the Invention
The present invention relates to a method for controlling a light spot on a recording layer of an optical disk.
2. Description of the Background Art
FIG. 10 denotes an example of a circuit configuration for realizing focus pull-in to a recording layer of optical disk such as a CD, DVD or the like. In the figure, reference numeral 0 denotes a light emitting optical system containing a semiconductor laser diode (LD), reference numeral 1 denotes an optical disk, 2 an objective lens, 3 a focus actuator which is rigid-body-like connected to the objective lens 2 and disposed in a magnetic circuit, 4 a half-mirror, 5 a photo-electric conversion element, 6 a focus error detector, 7 a phase compensator, 8 a change-over switch, 9 a driver amplifier, 10 a search waveform generator, 11 a zero-crossing detector for a focus error signal, 12 an AND circuit taking a logic product of a second control signal E from an external host device such as a microcomputer or the like and an output of the zero-crossing detector 11 and 13 an addition amplifier producing a whole addition signal.
FIG. 11 denotes a view showing an operation of each signal at a time of focus pull-in of a CD or DVD. In the figure, a symbol A denotes a focus drive signal which is output from the driver amplifier 9 to be applied to the focus actuator 3, B a focus error signal output from the focus error detector 6 (a pointer which directs a positional relationship between the optical 1 and the objective lens 2), C a total additional signal which is output from the addition amplifiers 13 and sums up totally the photo-electric conversion signals obtained from each area of the photo-electric conversion elements 5, D a first control signal which a total addition signal C is over a predetermined threshold value S and becomes xe2x80x9cHxe2x80x9d at a zero-crossing point of the focus error signal, E a second control signal from the host device which is xe2x80x9cHxe2x80x9d when the focus control loop is closed and is xe2x80x9cLxe2x80x9d when it is open, F a third control signal which the change-over switch 8 selects the focus control signal in a case of xe2x80x9cHxe2x80x9d and selects the focus search signal in a case of xe2x80x9cLxe2x80x9d, T1 the zero-crossing point of the focus error signal and Sr 2 a threshold value of the total addition signal for detecting a valid focus error signal.
A description will be given to a general focus pull-in operation hereinafter. As usual, a pull-in range of a focus servo lies in a relatively narrow range of a several to twenty xcexcm. In order to pull-in a focus servo, it is necessary to shift the focus actuator 3 up to the pull-in range of the focus servo. For example, one Hz of saw tooth search waveform is output from the search waveform generator 10. When a system is not a mode which closes the focus control loop, as a second control signal E from a host device is xe2x80x9cLxe2x80x9d, a third control signal F becomes xe2x80x9cLxe2x80x9d and a focus search signal is selected at the change-over switch 8. In this case, a search waveform is input to the driver amplifier 9 to drive a focus actuator 3. Based on the search waveform, a focus search operation that an objective lens 2 comes close to an optical disk 1 or goes away therefrom is carried out.
A focus error signal is a signal based on a reflection light from the optical disk 1 which changes together with the focus search operation and a laser light which is projected from a semiconductor laser diode (LD) 0 and reflected at a recording surface of the optical disk 1, is received by the photo-electric conversion element 5 and detected by the focus error detector 6, for example, by using a well known astigmatism method. The focus error signal detected herein is output to a phase compensator 7, which is formed by a phase-lead filter which leads a phase of the band width near 1 KHz and a suitable servo gain is given here. After the focus error signal obtains an adequate phase and a gain characteristic, the signal will be output to the change-over switch 8 as a focus control signal.
The focus error signal is also output to the zero-crossing detector 11. The zero-crossing detector 11 outputs a first control signal D that, when a signal level of the whole addition signal is over a predetermined threshold value Sr2 and the focus error signal detects a zero-crossing point T1, the zero-crossing detector 11 becomes xe2x80x9cHxe2x80x9d, while when a signal level of the whole addition signal is less than a predetermined threshold value Sr2, the zero-crossing detector 11 becomes xe2x80x9cLxe2x80x9d.
In this case, if the system is in a mode which closes the focus control loop, as a second control signal E from the host device becomes xe2x80x9cHxe2x80x9d and a third control signal F from the zero-crossing detector 11 is valid, a change-over switch 8 selects a focus control signal at a zero-crossing point T1 of the focus error signal and the focus actuator 3 is activated. Thus, a pull-in operation for the focus control is effected.
When a rotational drive of a disk is generally activated, a face deflection occurs. An absolute quantity of the face deflection is decided by a standard of each disk. As for DVD, xc2x1300 xcexcm is a standard limit. For example, when a DVD of the standard limit is driven by one double velocity, a rotational speed at the most internal circumference part of a disk is about 23 Hz, and suppose that an amount of the face deflection quantity is proportional to a radius of the disk, the face deflection is about xc2x1125 xcexcm at the most internal circumference, and the maximum face deflection velocity is about 18 mm/sec according to the following expression.
V=Axc2x7xcfx80xc2x7fxc2x7cos(2xcfx80xc2x7fxc2x7t)
where V is the face deflection velocity, A is an amount of face deflection and f is a rotational frequency.
On the other hand, since a speed of focus search is generally 10 mm/sec, the relative velocity between an objective lens and an optical disk is 30 mm/sec at the most. (This corresponds to a sum of the largest face deflection and the focus search speed.) As a rule, a pull-in capability of focus control depends on a phase compensation filter, a setting amount of gain, a sensitivity of the focus actuator or the like, and is 30 mm/sec, more or less. Since the pull-in capability exceeds the largest relative velocity of the objective lens and optical disk in all the region, it is possible to close a focus control loop.
Recently, a reproduction speed of the optical disk device became higher year by year and accordingly, the rotational speed of the disk has been made higher. For example, at the most internal circumference of CD, it is 16 times, and at that of DVD, the rotational speed is about four times. Considering a case of four times of DVD reproduction speed, and if the face deflection amount of the disk is xc2x1300 xcexcm in the standard limit, the largest face deflection velocity at the most internal circumference part is about xc2x170 mm/sec, and the maximum relative velocity is 80 mm/sec, where the speed for focus search is 10 mm/sec. As described above, since a possible velocity for pulling-in of the focus control system is usually 30 mm/sec, as shown in FIG. 12, there is very little region that the relative velocity of the objective lens and the optical disk is less than 30 mm/sec and it is seen that it is quite difficult to pull-in the focus control.
FIG. 12 shows a timely variation of the relative velocity between the optical disk and the objective lens and a horizontal axis shows a time(sec) and a vertical axis a relative velocity (mm/sec). A thick line at 10 mm/sec of the relative velocity shows the focus search velocity. An area between 10 mm/sec to xc2x130 mm/sec shows an area which the focus servo system can pull-in.
As described above, according to the prior art focus servo pull-in method, when the face deflection of the disk is large, the reproduction speed is fast, it often fails in the pull-in operation of servo. Therefore, it is a serious problem that as things stand, scarcely any servo can pull-in.
According to a first aspect of the present invention, a focus pull-in method for pulling-in a light spot irradiated through an objective lens onto a recording layer of an optical disk (1) using a focus actuator (3) which can move a position of the objective lens (2) comprises the steps of: (a) applying a search voltage to the focus actuator (3) to drive the objective lens to a direction for approaching to a focusing plane of the optical disk; (b) detecting whether or not a focus error signal (B) which is based on a reflected light from the optical disk attains a first signal level; (c) applying a braking signal indicating a stop of operation to the focus actuator, when it is detected in the step (b) that the focus error signal reaches the first signal level; and (d) starting the predetermined focus control, when a zero-crossing of the focus error signal is detected in the term for allowing the control start.
Preferably, according to a second aspect of the present invention, in the focus pull-in method of the first aspect, the step (b) is performed in which the first signal level is set to a quarter extent level of a peak value of the focus error signal or several times extent of a noise component superposed on the focus error signal.
Preferably, according to a third aspect of the present invention, in the focus pull-in method of the first aspect, the step (c) includes a step of applying the braking signal, when it is detected that all addition signals based on a reflected light from the optical disk exceed a second signal level and the focus error signal reaches the first signal level in the term for allowing the control start.
Preferably, according to a fourth aspect of the present invention, in the focus pull-in method of the first aspect, the braking signal applied by the step (c) indicates a stop of operation at the energy such that the maximum relative velocity between the objective lens and the optical disk can be reduced to the pull-in limit velocity for the predetermined focus control.
Preferably, according to a fifth aspect of the present invention, in the focus pull-in method of the first aspect, the step (c) includes a step of stopping the application of the braking signal, when a reversal of relative velocity between the objective lens and the optical disk is detected in the course of applying the braking signal.
Preferably, according to a sixth aspect of the present invention, the focus pull-in method of the first aspect further comprises a step of: (e) obtaining the relative velocity between the objective lens and the optical disk, and the step (c) includes a step of controlling an output time and/or an output level of the braking signal in response to the relative velocity obtained by the step (e).
Preferably, according to a seventh aspect of the present invention, in the focus pull-in method of the sixth aspect, the optical disk includes a first and second recording layers, and the step (e) includes a step of obtaining a first detection time of the focus error signal detected with respect to the first recording layer and a second detection time of the focus error signal detected with respect to the second recording layer, to obtain the relative velocity based on a time difference between the first and second detection times.
Preferably, according to an eighth aspect of the present invention, an optical disk device having a focus control loop and a focus search loop comprises an objective lens (2) for forming a light spot on a recording layer of an optical disk (1); optical detection means (5, 13, 6) receiving a reflected light from the optical disk for outputting a focus error signal based on the received reflected light; a search waveform generation means (10, 15, 8) for generating a search voltage for driving the objective lens to a direction which approaches to a focusing plane of the optical disk in response to a focus search instruction; a braking signal generation means (14, 16) for generating a braking signal indicating a stop of operation to the focus actuator, when it is detected that the focus error signal reaches the first signal level during a term for allowing a control start of a predetermined focus control; a zero-crossing detector (11) for detecting that a focus error signal reaches a state of zero-crossing; and means (12, 8) for reopening the predetermined focus control, when the zero-crossing detector detects zero-crossing during the term for allowing the control start.
Preferably, according to a ninth aspect of the present invention, in the optical disk device of the eighth aspect, the first signal level includes a quarter extent level of a peak value of the focus error signal or several times extent level of a noise component superposed on the focus error signal.
Preferably, according to a tenth aspect of the present invention, in the optical disk device of the eighth aspect, the optical detection means comprises: a photoelectric conversion element (5) for providing a photoelectric conversion signal by photoelectric conversion of the reflected light; and an addition amplifier (13) for totally adding the photoelectric conversion signal obtained from each area of the photoelectric conversion element to obtain all addition signals, and the braking signal generation means generates the braking signal, when the all addition signals exceed a second signal level and the focus error signal reaches the first signal level during the term for allowing the control start.
Preferably, according to an eleventh aspect of the present invention, in the optical disk device of the eighth aspect, the braking signal includes a signal that indicates a stop of operation at the energy which can reduce the maximum relative velocity between the objective lens and the optical disk to a pull-in limit velocity of the predetermined focus control.
Preferably, according to a twelfth aspect of the present invention, the optical disk device of the eighth aspect further comprises: a braking signal suspension means (17 to 19) having a relative velocity reversal detector (17) for detecting a reversal of the relative velocity between the objective lens and the optical disk, based on the focus error signal, for stopping an application of the braking signal, when the relative velocity reversal detector detects a reversal of the relative velocity during an application of the braking signal.
Preferably, according to a thirteenth aspect of the present invention, the optical disk device of the eighth aspect further comprises: a relative velocity reversal detector (10) for detecting a reversal of the relative velocity between the objective lens and the optical disk, based on the focus error signal, and the braking signal generation means controls an output time and/or an output level of the braking signal in response to the relative velocity.
Preferably, according to a fourteenth aspect of the present invention, in the optical disk device of the thirteenth aspect, the optical disk includes a first and second recording layer, and the relative velocity reversal detector obtains the relative velocity from a time difference between a first detection time of a focus error signal detected with respect to the first recording layer and a second detection time of a focus error signal detected with respect to the second recording layer.
According to the focus pull-in method of the present invention of a first aspect, before a predetermined focus control is performed, a braking signal is applied to make the relative velocity between the objective lens and the optical disk to reduce up to a velocity which a focus servo system including a focus actuator can pull-in entirely. Therefore, a face deflection of the optical disk is large and even when the reproduction velocity is fast, the pull-in of the focus servo which does not end in failure is realized.
According to the invention of a second aspect, since a first signal level which is a threshold level of a focus error signal is set to about one fourth of a peak value of the focus error signal or to several times of noise component which is superposed to the focus error signal, an erroneous generation of the braking signal can be prevented.
Further, according to the invention of a third aspect, since a braking signal is made to generate only when all addition signal exceeds a second signal level and the focus error signal searches to the first signal level, an operator does not mistake a timing for generating a braking signal, and a high precision focus pull-in ban be effected.
According to the invention of a fourth aspect, since said braking signal is made to generate, indicating a stop of operation at the energy such that the maximum relative velocity between the objective lens and the optical disk can be reduced to the pull-in limit velocity for said predetermined focus control, a reliable focus pull-in can be effected.
According to the invention of a fifth aspect, since a reversal of the relative velocity with an objective lens and an optical disk is detected, even if a brake is overpowered to cause the relative speed of the optical disk to reverse, the reverse is immediately detected to stop an application of the braking signal and a successful pull-in of the focus servo can be realizable.
Further, according to the invention of a sixth aspect, since an output time an/or an output level of said braking signal in response to the relative velocity is controlled and a braking signal of the optimum energy is applied to realize a successful pull-in of the focus servo. The relative velocity can be detected, for example, from a focus error signal detected in each layer of an optical disk having a plurality of recording layers.
According to the invention of a seventh aspect, from a time difference between a first detection time of a focus error signal which is detected by a first recording layer to be focused and a second detection time of the focus error signal which is detected by a second recording layer, since a method for computing the relative velocity is devised, the relative velocity can be detected with a simplified and low cost constitution.
In addition, the optical disk device of an eighth to fourteenth aspect can obtain the same effect as that of the focus pull-in method of a first to seventh aspect, repectively.
An object of the present invention to provide a focus pull-in method and an optical disk therefor in which even if the face deflection of a disk is large and the reproduction speed is fast, a focus servo can be certainly pulled in.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.